1. Field of the Invention
This invention relates to electronic instruments for detecting and measuring RF voltage wave signals on coaxial transmission lines, such as between a transmitting antenna and a transmitter. More particularly, the invention relates to an "insertion-type" RF wattmeter for detecting and measuring the forward and reflected voltage wave signals on a coaxial transmission line.
2. Description of the Prior Art
Insertion-type directional RF wattmeters are used in many applications in the RF field, particularly in matching antennas to coaxial transmission lines and in minimizing the voltage standing wave ratio (VSWR) on coaxial lines. Meters currently available for this application are, for example, of the type disclosed in U.S. Pat. Nos. 2,852,741, 2,891,221, 4,080,566, 4,075,554 and 4,263,653.
The principle of operation of these units is based on the use of a rigid, coaxial line section that is inserted in the coaxial transmission line, such as by standard coaxial connectors, and an inductive pickup loop or coil that is positioned in a transverse opening in the outer conductor of the line section. This pickup coil is adapted for rotation about an axis normal to the axis of the line section and is connected by special leads to a D'Arsonval meter movement. The resulting meter reading indicates the magnitude of the wave signal in watts, the indication being either that of the magnitude of the forward voltage wave level or the reflected voltage wave level, depending upon the particular orientation of the pickup coil.
The pickup coil is located in the electrical field between the inner and outer conductors of a coaxial transmission line and has a voltage induced therein proportional to the current I in the inner conductor, there being a mutual inductance M between the loop and the transmission line and the loop being positioned in the plane of the inner conductor of the line. A series circuit of resistance R and capacitance C connected across the transmission line conductors will give a voltage across the resistance R proportional to the voltage E between the line conductors. In directional couplers and so-called reflectometers, these arrangements are combined in a sampling circuit in which the resistor R is connected in series with the loop, and capacitive coupling is provided as by capacitive plates or windings on the loop and the inner conductor or by capacitive effects between the components of the sampling circuit and the inner conductor.
Considering this sampling circuit and using lumped impedances, it is apparent that the mutual inductance M is either positive or negative, depending upon the directional relation between the loop and the wave signal energy traveling on the line.
The instrument described obtains reversal of the mutual inductance M through 180.degree. rotation of the loop relative to the transmission line. The forward traveling wave has voltage E.sub.f and current I.sub.f, while the reflected traveling wave has voltage E.sub.r and current I.sub.r. Thus, if Z.sub.o is the characteristic impedance of the line, and p is the reflection coefficient, ##EQU1## where e is the total electromotive force induced in the loop or sampling circuit. The components are selected so that ##EQU2## K being a constant. If e.sup.+ is the electromotive force when M is positive, so that the voltage across R and the voltage induced in the loop are additive, and e.sup.- is the electromotive force when M is negative and the voltages referred to are opposed, the former gives a maximum and the latter a minimum indication, thus: ##EQU3## Thus the RF output voltage in the loop is directional and proportional to the voltage in the line due to either the forward or reflective wave, and from the loop voltage, measurements of the reflection coefficient and voltage standing wave ratio can be obtained.
It is also possible to measure the power P being fed through the transmission line ##EQU4##
The prior art instruments utilizing the principles referred to above generally included a coil physically inserted in a line section of suitable size with the coil rotatable through 180.degree. of travel in the field between the outer conductor and inner conductor of the line section in order to sense the magnitude of either the forward voltage wave level or the reflected voltage wave level. The coil was then connected to a conventional analog meter movement with the meter calibrated in watts to give a visual power indication representative of either the forward or reflective voltage wave signals on the transmission line. A detector circuit comprising a diode was provided between the loop and the meter to rectify the wave signal from the coil.
The accuracy of such instrument has been limited by the fact that high speed diodes which form the detector are inherently nonlinear in their current-to-voltage response, and are considerably affected by temperature.